The crystal structures of titanium oxides have three types of a tetragonal rutile type of a high temperature form, a tetragonal anatase type of a low temperature form, and a rhombic brookite type. Among them, rutile titanium oxides have high refractive indexes and thus are used as refractive index adjusters.
For application in optical materials, not only a high refractive index but also sufficient transparency as a coating film is required. However, rutile titanium oxides are generally produced by a solid phase method in which amorphous titanium oxides or anatase titanium oxides are calcined at a high temperature, and therefore, the particle diameters increase, thereby the transparency is impaired.
A wet method enables low temperature synthesis, which readily produces fine particles as compared with the solid phase method requiring high temperature calcination. Examples of a method for producing a ruffle titanium oxide sol by a wet method include a method of causing a reaction under the coexistence of a titanium salt and a tin compound having a rutile structure.
As the method using a titanium salt and a tin compound, disclosed is a method for producing an aggregate of titanium oxide-tin oxide composite colloids at 50° C. to 100° C. by causing a strong acid salt of titanium to react with metal tin in the presence of hydrogen peroxide (see Patent Document 1). Also disclosed is a method for producing rutile titanium oxide fine particles by causing a titanium compound solution to react at a temperature from room temperature to 100° C. in a pH range of −1 to 3 under the coexistence of a tin compound having a Sn/Ti molar ratio of 0.001 to 2 (see Patent Document 2). As a method for causing a gel containing titanium atoms dissolved in hydrogen peroxide to react, disclosed are a method in which a hydrated titanium oxide gel dissolved in hydrogen peroxide is mixed with cation-exchanged potassium stannate, and the resultant mixture is heat-treated (see Patent Document 3), a method in which a titanium compound is reacted with a tin compound and ammonia to produce a gel, and the gel is dissolved in hydrogen peroxide and is subjected to hydrothermal treatment (see Patent Document 4), and other methods.